California Golden Trout and Climate Warming: How Hot Is Too Hot?

The California golden trout (CGT), Oncorhynchus mykiss aguabonita, is one of the few native high-elevation fishes in the Sierra Nevada. They are imperiled due to exotic trout, genetic introgression, and degraded habitat, and now face further stress from climate warming. Their native habitat on the Kern Plateau meadows includes stream areas impacted by cattle grazing. Areas with reduced streamside vegetation (willows or sedge) and widened channels with shallow stream depths experience the warmest water temperatures. Climate change will further compromise CGT and their habitat in stream areas still being grazed, because the warmer water temperatures predicted under most warming scenarios could increase to lethal levels. Yet little is known regarding the specific temperature tolerances of trout in the Sierra Nevada. In a current analysis of CGT meadow streams, I found that stream temperatures often reached 25^oC in degraded areas and many areas lack the resiliency to future warming. These high temperatures are reportedly lethal for salmonids, but may affect CGT in more subtle ways such as growth, condition, or long-term survival. An important future research question is determining how hot is too hot for CGT.  To prepare for future stream warming, comparative studies are needed to determine differences in growth, condition, and survival in areas of varying temperature and determine specific temperature tolerances for the CGT.